Present-day optical data transport networks have high capacity and are very flexible in terms of allowing for different payload types and data rates. The combination of high capacity and high flexibility, however, gives rise to routing problems if data, for whatever reason, requires rerouting from its original path along the network. Specifically, because optical networks can carry large amounts of data at various speeds and sizes, present methods of rerouting are incapable of quickly and efficiently rerouting this data.
Within the field of optical networking, various standards exist for interfacing optical telephone networks. In North America, the SONET (Synchronous Optical Network) standards are used, while in Europe and most of Asia, the SDH (Synchronous Digital Hierarchy) standards are used. The SONET standards, and their SDH analogs, are used for interfacing equipment from different vendors. The SONET and SDH standards are similar; and for the purposes of this document, the term SONET includes both the SONET and SDH standards.
Several advantages are derived from using SONET. One advantage is that proprietary protocols for fiber-based digital transport have essentially been eliminated. SONET is based on the principle of direct synchronous multiplexing, which allows separate, slower signals to be multiplexed directly onto higher speed SONET signals without intermediate stages of multiplexing. Additionally, SONET provides advanced network management features, using nearly 5% of the total bandwidth. The SONET protocol is described in American National Standard for Telecommunications—Digital Hierarchy—Optical interface rates and formats specifications (SONET), ANSI T1.105-1991, which is hereby incorporated by reference.
A SONET protocol stack consists of the following four layers: the photonic layer, the section layer, the line layer, and the path layer. The photonic layer relates to converting electrical signals to optical signals. The section layer relates to the transport of STS-n (Synchronous Transport Signal) frames across the physical medium. Functions include framing, scrambling, section-error monitoring and communicating and adding the section-layer overhead.
The line layer allows the path layer payload to be transported, and it provides synchronization and multiplexing for the path layer. A line is the medium required to transmit data from the originating equipment to the terminating equipment. Finally, the path layer deals with the transport and mapping of services between path terminating equipment. These services include, but are not limited to, DS1, DS3 and video. The path layer carries information for mapping these services into an STS frame.
Providing end-to-end service requires fast service provisioning, maintenance, and quality assurance. The layered architecture in SONET helps a network operator to achieve service objectives for all service paths originating and terminating within the service provider's network domain. Some SONET signals, however, originate and terminate outside a network operator's domain. The network operators do not have access to the path-terminating points for such service signal. Thus, to meet end-to-end service objectives to manage all paths within the interface originating from all inter-network paths, an optional intermediate layer called Tandem Connection Overhead has been defined in the standards. This optional Tandem Connection Overhead layer exists between the line layer and the path layer, and is a standard specified in American National Standard for Telecommunications—Synchronous Optical Network (SONET) Tandem Connection Maintenance, ANSI T1.105.05-1994, hereby incorporated by reference. As discussed in the standard, the Tandem Connection Overhead layer deals with the reliable transport of path-layer payload and its overhead across a network. The use of Tandem Connection is application specific and at the discretion of the carrier.
A Tandem Connection is defined in the standard as a group of N STS-1s (N is any of the allowed line rate values) that are transported and maintained together through one or more tandem line systems, with the constituent SPE (Synchronous Payload Envelope) payload capacities unaltered. Tandem Connection maintenance can be performed in a single STS-1 (where STS is the digital version of the OC standard) or on a bundle with a capacity of N STS-1s, where N is any of the allowed line rate values. The size of bundles supported is application-specific and depends on the equipment used.
Before SONET was used, Plesiochronous Digital Hierarchy (PDH) networks existed with their own type of multiplexing. The types of multiplexing performed, however, placed severe restrictions on how a high-capacity pipe could be used to transport a variety of lower order digital pipes. In the pre-SONET world (including PDH), there was only one type of signal: the DS3 signal at 45 Mbit per second. In this signal, there is no standard overhead frame. Thus, if there is a failure in the network, only one type of signal with one type of payload at one type of rate needs to be multiplexed and sorted. If, however, a variety of signals, payloads and rates exist in a SONET environment, this multiplexing and sorting is extremely difficult, and can make it very difficult to reroute data in a practical way. In this scheme, if there is a network failure, rerouting and recreating the data is extremely difficult because within a few milliseconds, the system must figure out the slots needed for alternate routes, and then must assign those slots for alternate payloads.
It is clear from the above discussion that the desirable characteristics of high capacity and flexibility provided by optical networks creates efficiency problems if data on an optical network need to be rerouted.